The goal of the proposed research is to understand mechanisms utilized by the Drosophila Polycomb group (PcG) repressor protein complexes. Studying these repressors will provide information at the molecular level about how chromatin-associated proteins, and complexes, affect chromatin structure to repress gene transcription. Mammalian PcG homologs have been implicated in the control of cell proliferation and differentiation of hematopoietic cells. Studying the fly proteins may provide knowledge of processes that are linked to certain types of human cancers. There are three specific aims in this proposal. First, the complex containing the Drosophila repressor protein sex comb on midleg (SCM) will be isolated and characterized. Affinity chromatography will be used to isolate the major SCM complex using an epitope-tagged SCM protein. Mass spectrometry and/or automated Edman degradation will be used to obtain peptide sequences to identify unknown complex constituents. Second, biomolecular activities associated with the complex will be identified. Purified SCM complex will be tested for chromatin modification and remodeling activities in vitro. Third, additional Drosophila proteins which have a similar domain organization as SCM will be characterized. We will use molecular techniques to develop antibodies to these SCM-like proteins and determine if they co-localize with SCM, or if they are involved in different chromatin-associated processes.